1. Field of the Invention
This invention relates to an improved method for producing ethylene copolymer. More particularly, the present invention relates to a method for producing ethylene copolymer containing unsaturated glycidyl monomer as one component. Furthermore, the invention relates to the prevention of the occurrence of polymerization of an ethylene mixture containing unsaturated glycidyl monomer in a compressor and the piping system from the compressor to a reactor.
The ethylene copolymer obtained by the method of the present invention is suitable for use in preparing adhesives and coating materials. In addition, this ethylene copolymer can be mixed with other resins while retaining the excellent characteristics of polyethylene to provide useful resin compositions.
2. Description of Prior Art
The high pressure polymerization methods of ethylene copolymer containing unsaturated glycidyl monomer as one component are well known in the art according to Japanese Laid-Open Patent Publication Nos. 48-11388 and 48-89236, and Japanese Patent Publication Nos. 52-28836 and 58-55964.
For example, in Japanese Laid-Open Patent Publication No. 48-89236, methods for preparing copolymers consisting of three component materials of ethylene, vinyl acetate, and glycidyl methacrylate are disclosed. According to the disclosure in the former Japanese Laid-Open Patent Publication No. 48-89236, in the case that polymerization is carried out in a reactor, especially in a tubular reactor, the polymerization is done by adding 100 ppm to 1000 ppm of polymerization inhibitor to the solution of glycidyl ester, otherwise the blocking of polymerization tubes is caused to occur When the glycidyl ester is employed intact and continuous production of copolymer becomes impossible. Disclosed in Japanese Patent Publication No. 52-28836 is a method for preparing ethylene copolymer which is characterized in that one or more kinds of unsaturated glycidyl monomers are mixed into the gas that is compressed to reaction pressure and polymerization is then carried out.
In the method described in Japanese Laid-Open Patent Publication No. 48-89236, however, physically and chemically unstable polymer mainly consists of unsaturated glycidyl monomer deposits on cylinder packing portions of an ultra high pressure compressor which compresses it to a reaction pressure, disabling the long period operation of the reaction. Meanwhile, in the method described in Japanese Patent Publication No. 52-28836, because one or more kinds of glycidyl monomers are mixed into the gas that is compressed to reaction pressure, an expensive ultra high pressure pump is required which is uneconomical.
In the case when the conversion rate in a reactor is low, the concentration of unsaturated glycidyl monomer in the unreacted recycle gas that is separated by a high pressure separator, is raised. So that a polymer mainly consisting of unsaturated glycidyl monomer is produced in a compressor and the long period operation becomes impossible inadvantageously.
In Japanese Patent Publication No. 58-55964, a method for producing ethylene polymer or copolymer is disclosed. This method is characterized in that lubricating oil containing 0.1 to 10 ppm of a polymerization inhibitor relative to the quantity of ethylene or the mixture of ethylene and ethylene-copolymerizable comonomer, is used. The undesirable thermal polymerization in the compressor can be avoided by this method. However, the quantity of the lubricating oil discharged outside the system as the drain of the compressor is large because the molecular weight of lubricating oil is generally high, and when the reactor is a tubular type one, the method cannot be said satisfactory for avoiding undesirable thermal polymerization in the preheating zone which heats the mixture of ethylene and unsaturated glycidyl monomer to reaction temperature. Furthermore, it is necessary to avoid the polymerization in the feeding line and feed pump prior to the reaction system because the polymerization of unsaturated glycidyl monomer is liable to occur; however, this method is not effective at all in this regard.
Japanese Patent Publication No. 59-80407 (claiming Convention Priority of U.S. Ser. No. 426,409; Sept. 29, 1982) or British Patent Application No. 2,128,198A discloses a high pressure polymerization process which comprises the step of compressing monomer feed before introducing it into a polymerization zone wherein it undergoes high pressure polymerization wherein the monomer feed is compressed to reaction pressure in a compressor having at least one compression stage and at least one lubricated compressor cylinder assembly provided with a lubricant containing from 3 wt. % to 10 wt. % of an antioxidant, thereby reducing gas leakage from the compressor. The undesirable thermal polymerization in the compressor can be avoided by this method; however, the method is not satisfactory in order to avoid the undesirable thermal polymerization in the preheating zone in which the mixture of ethylene and unsaturated glycidyl monomer is heated to reaction start temperature.
Furthermore, Japanese Laid-Open Patent Publication No. 61-127709 discloses that the void portions of packing and retainer of an ultra high pressure pump for comonomers are filled with grease-like oil which contains 2% to 20% by weight of a polymerization inhibitor and, if necessary, 100 to 5000 ppm (weight basis) of a polymerization inhibitor is added to the lubricating oil for the ultra high pressure pump.
By these methods, however, it is not possible to solve effectively the problems to avoid the thermal polymerization in a high pressure compressor, in the outlet piping of the compressor and in the preheating zone and to reduce the retarding effect of the polymerization inhibitor in the reaction vessel.